Re: Mistakes in commit log messages

[Jim Porter <jporterbugs@gmail.com>]

On 4/11/2023 11:45 AM, Eli Zaretskii wrote:
>> Date: Tue, 11 Apr 2023 11:31:55 -0700
>> Cc: emacs-devel@gnu.org
>> From: Jim Porter <jporterbugs@gmail.com>
>>
>> Finally, I think it would make sense to have this be a purely advisory
>> warning for now so that we could check it into the Emacs tree soon-ish.
> 
> How will this help, given that some people use Magit to commit and
> some use VC, not the Git commands from the shell?  Are we sure the
> warnings will be seen, let alone acted upon?

I don't know. If VC and Magit don't show messages from Git hooks, they 
probably should. Those messages are there for a reason, after all. 
However, warnings that only some users see would hopefully still help us 
get feedback on whether the validation works properly.

Some miscellaneous thoughts on this:

One issue with the current implementation (mentioned elsewhere in this 
thread) is that it doesn't work for deleting a file from the repo. 
Fixing this in the commit-msg hook is tricky. We can get the list of 
changed files via "git diff --name-only" (and then we could compare our 
commit message against that list), but there's a problem: I don't know 
of a good way to detect when the user is amending a commit[1]. For a 
normal commit, you'd get the changed files via something like "git diff 
--staged --name-only", but for an amended commit, you'd want to add 
"HEAD^" to that command.

Another option might be to do this as a pre-push hook. By then, the 
commits are finalized, so we can more easily compare the commit message 
and its diff. This is also nice because when the commit-msg hook errors 
out, it throws away the commit message; quite annoying if you just made 
a small typo! However, it does mean that committers would need to be 
comfortable with amending their commit messages if they hit this error.

(We could also check this in a post-commit hook so that committers are 
alerted *before* they try to push, but post-commit hooks can't abort the 
commit: it's already done! Maybe we could have a post-commit hook that 
warns the committer, and a pre-push hook that actually errors out.)

[1] The only way I've seen to do this is to look at the arguments from 
the parent (Git) process that called the hook: 
<https://stackoverflow.com/questions/19387073/how-to-detect-commit-amend-by-pre-commit-hook>. 
I'm not sure how reliably that works, especially on platforms like 
MS-Windows...